1. Field of the Invention
The present invention relates to a keyboard switch which is used for a data inputting device, such as a keyboard for an electronic instrument, and a method of manufacturing the same.
2. Prior Art
FIG. 6 is a schematic longitudinal cross-sectional view showing an embodiment of a conventional keyboard switch.
In FIG. 6, the keyboard switch 40 is mainly composed of a membrane contact point sheet 41, a push button key 42 and a reinforcing plate 43.
More in detail, the membrane contact point sheet 41 is composed of integrally laminated flexible thin plates of an upper contact point film 44, a spacer 45 and a lower contact point film 46. The membrane contact point sheet 41 has a plurality of contact portions 47 at positions where the push button keys 42 are mounted. A single contact portion and a single push button are illustrated in FIG. 6. A through hole 48 is formed outside the contact portion 47 on the membrane contact point sheet 41.
The membrane contact point sheet 41 is mounted on the reinforcing plate 43, on which a through hole 49 is formed so as to align with the through hole 48 near the contact points 47. The push button key 42 is composed of a housing 50, a key top 51, a return spring member 52 and a pressing spring member 53. The housing 50 is composed of a hollow lower housing portion 50a having an opening at the lower surface thereof and an upper housing portion 50b which projects upward from the upper surface of the lower housing portion 50a. Provided at the center of the upper housing portion 50b is a through hole 54 which communicates with the inside of the lower housing portion 50a. Furthermore, provided on the lower surface of the lower housing portion 50a is a threaded hole 55 used with the hole 48 of the membrane contact point sheet 41. The key top 51 comprises a key lever portion 51a slidably inserted into the through hole 54 of the upper housing portion 50b and a key top portion 51b integrally formed on the upper end of the key lever portion 51a. Provided between the key top 51 and the housing 50 is a retaining pawl, not shown, for preventing the key top from dropping out of the housing 50.
This push button key 42 is assembled as follows. The spring member 52 is arranged around the periphery of the upper housing portion 50b. Then the key lever portion 51a is inserted into the through hole 54 while compressing the spring member 52. When it is inserted into a given depth so that the retaining pawl enters the lower housing portion 50a, the retaining pawl retains the lower housing portion 50a so as to prevent the key lever portion 51a from dropping out of the lower housing portion 50a. Thereafter, the spring 53 for pressing down the contact point 47 is mounted on the lower end of the key lever portion 51a projecting into the lower housing portion 50a.
Then the push button key 42, the membrane contact point sheet 41 and the reinforcing plate 43 are combined to assemble the keyboard switch. Namely, the membrane contact point sheet 41 is arranged on the reinforcing plate 43. Thereafter, the push button key 42 is arranged on the membrane contact point sheet 41 so as to align with the contact portion 47. At this state, a screw 56 is screwed into the threaded hole 55 through the through holes 49 and 48. As a result, the membrane contact sheet 41 is fixedly mounted on the reinforcing plate 43 and at the same time the push button key 42 is fixedly mounted on the membrane contact sheet 41.
Consequently, the keyboard switch 40 constructed as set forth above requires the upper housing portion 50b a certain length DO to guide the key lever portion 51a.
In order to reduce the entire height of the keyboard switch 40, it is necessary to reduce the stroke of the key top 51 or shorten the length DO of the sliding portion by reducing the height of the key lever portion 51a or the housing 50. However, since the appropriate stroke is supposed to be 3 to 5 mm, in this kind of keyboard to thereby limit the reduction of the stroke, it has been difficult to realize a keyboard switch having enough key top stroke, an excellent pressing touch and less entire height, and the keyboard switch has already reached its structural limit.